A face that shows eagerness or being cocky. It also shows as a sign of arrogance or confidence in a given situation. Or it simply means that they are humorously (if not seriously) plotting something.

Gamer 1: Hey, you know what we should do on MGO? We gotta try that CQC and C4 combo!
Gamer 2: Hmm... yeah, we should. >:-)
Gamer 1: Got any other ideas?
Gamer 2: We can knock out the the guy we planted the C4 on it too. We use him as bait to lure someone into trying to wake him up... and then, BOOM! Two birds with one stone! GENIUS!!